There is a large variety of requirements for the next generation of mobile communications system (5G) implying that frequency bands at many different carrier frequencies will be needed. For instance, lower frequency bands will be needed to achieve sufficient coverage and higher frequency bands (e.g. mmW, i.e. near and above 30 GHz) will be needed to reach the required capacity. At high frequencies the propagation properties are more challenging and beamforming might be required both at a base station and at a communication device (e.g. a user equipment, UE, used as example in the following) in order to reach sufficient link budget.
UE antennas at high frequencies are more directive than UE antennas at lower frequencies which means that a single antenna element at a high frequency typically does not offer omnidirectional coverage. Therefore, multiple antenna elements with beam patterns pointing in different directions and with different polarizations are preferred at the UE in order to improve the link budget and to offer omnidirectional coverage.
There are in essence three different implementations of beamforming done at baseband that are being discussed for the UE: analog beamforming, digital beamforming and hybrid beamforming. Each implementation has its advantages and disadvantages. Digital beamforming is the most flexible solution but is also the most costly due to the higher number of required hardware components compared to analog solutions (e.g. radio frequency chains and baseband chains). Further, the high number of hardware components and the size associated thereto are drawbacks adding cost to the UE. The analog beamforming solution is the least flexible one but UE implementing such solutions are less costly to manufacture owing to a reduced number of radio- and baseband chains compared to the digital solution. Hybrid beamforming is a compromise between the analog and digital beamforming. Depending on cost, power consumption, and performance requirements of different UEs, different implementations will be needed. Due to hardware constraints at high frequencies (mmW), the most likely implementation in these frequency bands is analog beamforming.
At high frequencies, a conceivable UE beamforming implementation is to have a single baseband chain that is switched between several directional antennas. This would give an even less complex solution than the analog solution mentioned above. This is a way to achieve beamforming gain with omnidirectional coverage, although not simultaneously, at a low cost. A difficulty with this approach is to determine which antenna to use. Switching the baseband chain to different antennas in order to find the best one will lead to reduced data throughput since some (or all) antennas that are tested in the switching will have lower received power than the currently used antenna.